In controlling the transmission of fluids in industrial processes such as, for example, oil and gas pipeline distribution systems, chemical processes, etc., it is often necessary to transmit the process fluid at a relatively high pressure through portions of the distribution system or process demanding a high volume or flow rate of the process fluid. As the high pressure process fluid travels through the distribution system or process, the pressure of the process fluid may be reduced at one or more points to supply a lower volume of the process fluid at a lower pressure to a sub-system that uses or consumes the process fluid.
Pressure reducing fluid regulators are typically used to reduce and control the pressure of a process fluid. In general, a pressure reducing fluid regulator varies the restriction through a valve that is serially interposed in the fluid flow path. In this manner, the pressure reducing fluid regulator can control the fluid flow rate and/or pressure provided at a downstream outlet of the regulator. Pressure reducing fluid regulators are typically implemented using either a pilot operated control mechanism or a direct acting control mechanism.
Pilot operated fluid regulators typically include a pilot stage diaphragm having a relatively small surface area. The pilot stage diaphragm typically responds to regulator output pressure to drive a second or main control stage that employs a diaphragm having a relatively large surface area. The larger diaphragm of the main stage provides the large forces needed to actuate the regulator valve.
Direct acting fluid regulators eliminate the pilot stage so that the fluid output pressure typically acts on a single relatively large diaphragm that is directly coupled to the regulator valve. As a result, a direct acting fluid regulator may be provided in a relatively compact housing having a relatively small mounting envelope.
A wide variety of pressure reducing fluid regulators, each of which may have a different set of design features suitable for a different application, are commonly available. For example, pressure reducing regulators designed for use in controlling the pressure of natural gas at a consumer site (e.g., a residence or commercial building) or other custody transfer point, are typically required to be relatively accurate. High regulator accuracy is usually achieved by configuring the regulator to have a high proportional band gain (i.e., a high mechanical gain). Several factors may be varied to achieve a high proportional band gain. For example, the regulator diaphragm area and lever ratio (i.e., a unit amount of diaphragm travel divided by the amount of valve stem and disc travel produced by the unit amount of diaphragm travel) substantially control the proportional band gain of a regulator. Generally speaking, a larger diaphragm area generates larger forces at any given pressure drop across the regulator and, thus, allows a corresponding reduction in the lever ratio. A reduction in the lever ratio results in a higher proportional band gain for the regulator, which increases the accuracy with which the regulator can control its output pressure.
On the other hand, pressure reducing fluid regulators designed for use in controlling the distribution of liquefied petroleum (LP) gas are relatively compact, which enables these regulators to be more easily mounted in confined spaces (e.g., tank domes). Accuracy is not as important for LP gas applications as it is for natural gas applications. Thus, a relatively smaller diaphragm can be used to minimize the mounting envelope of LP gas regulators. In addition, because pressure reducing regulators used in LP gas applications are often required to control relatively large pressure drops, the lower proportional band gain of these regulators tends to reduce the instability problems that are common in these applications.
Thus, the different performance requirements associated with different pressure reducing regulator applications have historically been in tension. The required design tradeoffs resulted in different regulator designs for different applications. For example, the use of a pressure regulator designed for use in a natural gas system is typically not suitable for use within a LP gas system due to the relatively large mounting envelope of a natural gas regulator. Further, the relatively larger proportional band gain of a natural gas regulator aggravates instability problems typically associated with the large pressure drops often encountered in LP gas applications. Likewise, due to their relatively low proportional band gains, pressure reducing regulators designed for use with LP gas systems are typically not sufficiently accurate for use in natural gas systems.